Currently the best performing styrenic block copolymer for hot-melt pressure sensitive adhesive applications is styrene-isoprene-styrene (see, U.S. Pat. No. 7,307,124). Another good styrenic block copolymer for hot-melt pressure sensitive adhesive is styrene-butadiene/isoprene-styrene where the midblock is a random mixture of butadiene and isoprene. These copolymers have the best performance because they have good adhesive properties, good tensile properties and acceptable hot-melt viscosity when modified with a tackifier.
The petro derived monomers like isoprene and butadiene suffer from price volatility and recurrent shortages. In this regard it is important to consider alternate bio-derived monomers which can help reduce the need for petro-derived monomers. One such class of alternates is terpenes which are naturally occurring products that include myrcene (C10H16). Terpenes such as myrcene can also be biosynthesized on an industrial scale from naturally occurring products.
U.S. Pat. No. 4,374,957 to Quirk discloses a linear triblock polymer of styrene-myrcene-styrene (SMS) made by sequential polymerization with an organolithium initiator. In that work it was noted that neat, non-formulated styrene-butadiene-styrene and styrene-isoprene-styrene block copolymers lacked the tackiness to make them useful as adhesives. Inventive SMS block copolymers were made. The polystyrene content of the examples ranged from 23 to 58 wt. % of the SMS block copolymer and the resulting tensile strengths ranged from 3.5 to 12.8 MPa. No formulated adhesive compositions were made from those block copolymers. Further, while it was qualitatively observed that the inventive polymers had increasing tackiness as the myrcene content increased, no adhesive properties were analytically measured on these non-formulated polymers and as such the polymers themselves were not evaluated as adhesives.
The need exists for styrenic block copolymers that have properties similar or superior to styrene-isoprene-styrene block copolymers especially when used in adhesive compositions. In particular there is a need for styrenic block copolymer with a low hot-melt viscosity and a melt flow rate that is superior to styrene-isoprene-styrene block copolymers to allow for a lower temperature application of the adhesive. Hot-melt viscosity stability is also a highly desired feature because viscosity changes are minimized making the hot-melt process more robust and reducing waste during production. Having such attributes, the adhesives of the present invention may be applied to a desired substrate by any method known in the art and include, without limitation, roll coating, slot coating, swirl spraying, spraying.